Finish up function call implementation, it appears to be working

Functions are downcasted to a `Fun` trait, which will construct the
appropriate stack frame.

A few other things have been shifted around that affect internal APIs
while things are still under construction.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/mod.rs

@@ -23,10 +23,14 @@ impl Compile {
     /// Compiles the given AST body.
     pub fn compile(mut self, body: &Body) -> error::Result<(ConstPool, UserFunRef)> {
         self.push_scope_layer();
-        let main = thunk::CompileBody::new(&mut self)
+        let mut main = thunk::CompileBody::new(&mut self)
             .compile(body)?
             .flatten()
             .to_vec();
+        // XXX TODO(compile)
+        // remove this when we get returns implemented
+        main.push(crate::vm::inst::Inst::PushSym(crate::obj::reserved::NIL_NAME.sym));
+        main.push(crate::vm::inst::Inst::Return);
         let globals_syms: std::collections::BTreeMap<_, _> = self.pop_scope_layer().unwrap()
             .into_iter()
             .map(|(sym, name)| (name, sym))